Considering this: https://community.ptc.com/t5/Mathcad-Tips/Official-PTC-Mathcad-15-and-Prime-1-6-End-of-Sale-Announcement/ta-p/818870
there are two challenges for you:
1.
Do you have the (full) installation source available? You cannot download it anymore from the PTC website.
2.
Do you have a perpetual license for it? If yes, contact PTC support (that's NOT this forum!) for help on transferring the license.
Success!
Luc